Oscillators which produce R.F. energy typically generate sideband noises, including thermal noise, shot noise, and flicker noise. This noise is typically random and is referred to as phase noise.
Spurious phase noise signals can also be generated. These consist of discrete signals appearing as distinct components and can be related to known phenomena in the signal source, such as power-line and/or vibration modulation. This spurious noise is also referred to as phase noise.
The presence of phase noise in R.F. signal sources is a concern in several applications, including applications related to frequency conversion, digital communication, and analog communication.
Phase noise is also of great concern in radar equipment, especially Doplar radar equipment which determines the velocity of a target by measuring the small shifts in frequency in the return echoes. When decorrelation techniques are used to measure this frequency shift, the presence of phase noise can partially or even totally mask the target signal.
In order to minimize the amount of phase noise generated by a particular system, it is highly advantageous to be able to measure that phase noise. As a result, there has been a continuing need for equipment which can accurately measure such phase noise.
The most straight forward technique is to connect the test signal to a spectrum analyzer, a device which directly measures the power spectral density of the oscillator. However, this method is limited by the dynamic range and resolution of the spectrum analyzer, as well as the phase noise which the spectrum analyzer itself produces. Moreover, it is difficult with this method to accurately measure phase noise which is close in frequency to the carrier signal.
There are also time-domain techniques which down convert the signal under test to an intermediate frequency. A high-resolution frequency counter is then used to take repeated measurements of the intermediate frequency signal. The frequency differences between the measurements are then used to compute the amount of the phase noise.
Time-domain methods, however, are not well suited for measuring phase noise having a frequency substantially different from the carrier frequency. It is also difficult to utilize such techniques for measuring phase noise that is flat or decreasing slowly in frequency.
Another technique is to demodulate the carrier on which the phase noise is present. However, this technique requires complex phase locking circuitry. Moreover, it does not work well with signal sources which are not constant in frequency.
A still further technique is to use a frequency discriminator which can be implemented in several ways, one of which is a delay line. With this technique, a delay line, phase shifter and detector are used to decorrelate the phase noise present in the R.F. signal source. This converts the phase noise into a baseband signal which can be conveniently measured on a spectrum analyzer.
One principal problem using delay line frequency discriminators are limitations imposed by the delay element.
When a waveguide is used as the delay element, for example, it is very difficult to obtain long delays such as 10 microseconds. Although long delays can be achieved with long lengths of coaxial cable, very high losses result, making measurement of low noise levels very difficult. In addition, coaxial cables are usually quite bulky, making it difficult to change cables when a different length delay is desired.
Another technique mixes the test signal to a lower frequency and then transports that lower mixed frequency signal through a digital, quartz, or surface acoustic wave (SAW) delay line. Such techniques, however, require complex synchronization circuitry, often introduce undesirable distortion, and are bulky.
A still further technique has been to use a cavity resonator as the delay element. This device, however, requires a very high Q to detect low level noise. Unfortunately, high Q resonators are very expensive and are very sensitive to vibration. They also require mechanical adjustment in order to match the desired frequency range.
In short, there has been a continuing need for a noise test set which can accurately measure low level phase noise, especially phase noise that is near in frequency to the carrier frequency.